LED Christmas Lights Conundrum!

[DetlefSchmitz]

I suppose that if there was a temporary bad connection after the forth led then the current through the first four would exceed the absolute maximum and destroy them.

 

[JohnW2]

I suppose that if there was a temporary bad connection after the forth led then the current through the first four would exceed the absolute maximum and destroy them.

Possibly - but (a) that bad connection would have to have somehow mended itself (and I've just had a good 'wiggle', and can't detect any intermittentcy of connection!) and, (b) by my calculation, even that would only result in about 23 mA through each of the LEDs, which I wouldn't have expected to be enough to 'instantly kill' any LED (if they lit at all when I tried them this year, it could only be for a fraction of a second, because I was unaware of them ever being 'on' from the moment I applied power).

Kind Regards, John

 

[DetlefSchmitz]

I'm sure you know best. However 7mA absolute maximum is not unusual for low current leds. And you wanted a single fault to cause exactly those 4 leds to break. Occam's razor.

 

[JohnW2]

I'm sure you know best. However 7mA absolute maximum is not unusual for low current leds.

Well, it sounds as if you are wrong in thinking that I know best, since you appear to know better I didn't recall having seen LEDs with a (short-term) absolute max current of much less than 100 mA (or, usually, a continuous maximum less than around 20-30 mA) - so I have to bow to your superior knowledge.

And you wanted a single fault to cause exactly those 4 leds to break. Occam's razor.

I did, and I still do (albeit purely as a matter of curiosity). It's my inability to reproduce a 'temporary bad connection' (even with what is, since I've tried again, some pretty violent 'wiggling' and shaking ) that makes me less than convinced by this particular suggestion is the answer in my case.

Kind Regards, John

 

[JohnW2]

Eureka, I think! After the below series of quotes is something approaching 'the answer' ....

maybe the conductors have a noticable resistance, and those ones were slowly cooking unnoticed last year as the voltage drop across the wires was making the closer ones take a lot more current. .... I didn't mean the conductors were cooked, I meant the voltage drop along the length of the cable would be greater as you get further away. So the LEDs which all have the same forward voltage, would get a greater current where the voltage is higher, i.e. close to the power source.

.... but maybe they were cracked or something and the storage let the dampness in (stored in the loft?)....

maybe the four that failed were from the same batch of defective manufacture, the quality control on these non critical mass produced elements has low stanfards.

I suppose that if there was a temporary bad connection after the forth led then the current through the first four would exceed the absolute maximum and destroy them.

Most likely those 4 have died as a result of excess current given they are closest to the supply. The cheap LEDs that tend to get used are particularly intolerant of thermal damage. .... Are they soldered directly to the wiring string?

come on John. Less typing, and more practical testing/disecting to find the cause!... I was thinking you would be able to cut 4 out and leave the rest working.

Right. I seem to have the answer (which does involve a fair bit of 'co-incidence') - and it would seem that JohnD v2.0 probably got closest to getting the prize (highlighted in red above). It might also have been what aptsys was thinking about when he asked about soldered connections, but he didn't say so ....

I've done as Andy suggested and cut off the first four LEDs (and their wiring) for inspection.

Somewhat to my surprise, the wire/cable is not too bad. It seems to be 7 x 0.15mm diameter copper. That's about 0.124mm² CSA which (for a pair of conductors) should amount to a resistance of about 355 mΩ/metre. The LEDs are 200mm apart, which means that, at the supply end of the chain, where the current will be greatest ( a bit under 100mA), the VD between two adjacent LEDs would be under 7 mV. Whilst I accept that there has to be a dividing line somewhere which represents 'the last straw', it seems fairly unlikely that a difference of voltages of <7mV between one LED and the next one would result in one, but not the other, failing.

In any event, that is moot , because it transpires that the LED elements (even though 'cheap and nasty') have not 'failed' (which puts paid to nearly all of the suggestions that have been put forward). Although it remains the case that the wires 'disappear' into a solid lump of plastic, it transpires that (once one has removed the coloured 'envelope') the actual LED elements are 'push in (hence answering aptys's question about soldering). When I pull them out, the 'push-in' contacts were seriously corroded (see example photo below) and, when I cleaned the contacts up, all the LED elements worked fine.

The corrosion might simply have been the consequence of storage in the damp cellar. However, some of these sets (I'm not sure of which) have, in past years, been used outdoors, so it is possible that the was some water ingress and that they were put away with water (or, at least, moisture) inside them.

So it seems that the mystery is almost solved. I certainly know why the four LEDs in question "weren't working", but I think I'm still going to have to accept the (what I would have thought very unlikely) co-incidence of this not only happening to four LEDs simultaneously, but to the first four in the string 'just due to chance'!

Kind Regards,
John

 

[bernardgreen]

I powered up a string of LED lamps this afternoon and found that most of them were not working. Very annoying as they are ( were ) used in a sign pointing to one of the Village Christmas Fayre locations. IThe whole thing needs to be rebuilt.

Several LEDs showed signs of corrosion / discolouring in the metal to plastic seal suggesting moisture had got into the LED along the lead frame.

 

[JohnW2]

I powered up a string of LED lamps this afternoon and found that most of them were not working. Very annoying as they are ( were ) used in a sign pointing to one of the Village Christmas Fayre locations. IThe whole thing needs to be rebuilt. .... Several LEDs showed signs of corrosion / discolouring in the metal to plastic seal suggesting moisture had got into the LED along the lead frame.

It sounds as if it's a case of 'join the club'

In my case I think that, rather than replace the whole set with one of my spare sets, I'm probably going to cannibalise one of my spare sets, and use just 4 of them to 'splice into' the faulty set to replace the four I've removed. I'll then have plenty of cannibalised 'spares' left for future 'repairs'!

Kind Regards, John

 

[JohnD]

I usually buy my Xmas lights on the 28th December.

Terrific bargains to be had.

 

[JohnW2]

I usually buy my Xmas lights on the 28th December. Terrific bargains to be had.

I think that I may well have bought these sets around that time of year, for the same reason - but that was several years ago!

My long-gone grandmother always used to do most of her Christmas-related shopping (cards, wrapping paper, decorations, presents, and even Christmas puddings) in early January! She also (much to the concern of all of us hungry people around her) used to toddle down to the local market late in the afternoon on Christmas Eve (when the market was 'packing up') to buy the (highly-reduced-in-price) turkey - and, fortunately, always got away with that!

Kind Regards, John